1. Field of the Invention
The present invention relates to a superconducting coil apparatus, and, in particular, to superconducting coil apparatus which has a plurality of formers concentrically disposed, each former having a superconducting wire wound around its outer surface in spiral.
2. Description of the Related Art
Super conductive coil apparatuses generally have a plurality of coil elements concentrically disposed, each element having a spiral wire, in order to be compact without reducing the electromagnetic force generated by the coil apparatuses. Unlike normal conductive coil apparatuses, however, superconducting coil apparatuses should have superconducting wires cooled by a cooling medium to be in a superconducting state. In this respect, conventional superconducting coil apparatuses are constituted as follows.
As shown in FIG. 1, a plurality of coil units 51 and 55 each having a coil element are concentrically arranged. The first coil unit 51, disposed at the center, comprises a cylindrical core member 52 and a plurality of plate-shaped spacers 53 arranged on the outer surface thereof to constitute a cylindrical member. A superconducting wire is wound in spiral around the outer surface of the cylindrical member constituted by the spacers, thus constituting a first coil element 54. The second coil unit 55, disposed outside the first coil unit 51, is similarly constituted. To be specific, the second coil unit 55 has a plurality of plate spacers 56 disposed around the outer surface of first coil unit 51 and constituting a cylindrical member. A superconducting wire is wound in spiral around the outer surface of this cylindrical member, thus constituting a second coil element 57. Though not illustrated, a plurality of other coil units are disposed outside the second coil unit 55.
In assembling a superconducting coil apparatus of this type, however, it is necessary to wind a superconducting wire around the outer surfaces of a plurality of spacers of each coil unit after sequentially arranging these spacers. This assembling is therefore trouble some. Further, the superconducting coil apparatus generates a significantly high magnetic field as compared with normal superconducting coil apparatuses, thus causing significant large force to act on the superconducting wire. This necessitates that the superconducting coil apparatus have very high mechanical strength. In the superconducting coil apparatus shown in FIG. 1, however, since a cylindrical member is constituted by a number of spacers arranged side by side in circle, this cylindrical member may be deformed by coil-generated electromagnetic force. In other words, the conventional superconducting coil apparatus does not have a sufficiently high mechanical strength. In addition, superconducting coil apparatuses are soaked in a cooling medium so that the superconducting wires are cooled by the cooling medium. In the coil apparatus shown in FIG. 1, however, the superconducting wires may not come in sufficient contact with the cooling medium.
The superconducting coil apparatus of FIG. 1 has various shortcomings as mentioned above. As a solution to the problems, there has been proposed a superconducting coil apparatus as shown in FIG. 2 (though this coil apparatus is not completed yet, nor has it been worked yet).
As shown in FIG. 2, first and second coil units 61 and 62 respectively have first and second cylindrical insulative formers 63 and 64 molded of resin. Each former has fluid passages 65 for passing a cooling medium and a plurality of recess portions 66 which, in association with one another, constitute a spiral groove. Second former 62 is disposed concentric to and outside first former 62, and outside the second former 62 are similarly disposed other formers (not shown) which are constituted in the same manner as the first and second formers. In assembling the superconducting coil apparatus, therefore, a superconducting wire is fitted in recess portions 66 to thereby be wound in spiral around formers 61 and 62. This facilitates the assembling work. In addition, the cooling medium flows through fluid passages 65 and comes in touch with the superconducting wire to thereby effectively cool the wire. As each former is molded into a cylindrical shape, it has a sufficiently high mechanical strength.
In assembling the superconducting coil apparatus, however, first former 63 should be inserted in second former 64 after the superconducting wire is wound around the former former 63, and this inserting work is very troublesome.
To facilitate the above inserting work, there should be a predetermined gap (play) provided between the outer surface of first former 63 and the inner surface of second former 64. When a number of formers are concentrically arranged, therefore, there need a number of gaps (plays) accordingly. This inevitably enlarges the coil apparatus.
Further, the superconducting wires are not wound around the axial-directional end portions of the first and second formers (the upper end portions of the winding frames in FIG. 2). When wound around the first and second formers, the superconducting wires are applied with predetermined tension (the reason for nonwinding at the end portions and this tension will be explained in the description of the preferred embodiments). In addition, the superconducting wire wound around first former 63 is also continuously wound around second former 64. For this purpose, second former 64 is provided with a groove 67 extending to an associated recess portion 66 from the axial-directional end portion (upper end portion). The superconducting wire extending from first former 63 and given with predetermined tension is guided downward from the upper end of this groove 67 to be fitted in the recess portion 66. When guided in this manner, the superconducting wire scrapes or touches the edge of groove 67 and may be damaged or cut in the worst case. Further, it is tiresome to guide the tensed superconducting wire along groove 67.
Furthermore, the electromagnetic force acting on the superconducting wire is separated into the compression force of the coil element in its axial direction and the warp force of the same coil element in its radial direction. Since the shape of the recess portions is not matched with that of the superconducting wire, the superconducting wire, when applied with the compression force, may be shifted in the axial direction. This causes friction between the superconducting wire and the outer surfaces of the first and second formers, thus generating heat. This heat causes transition of the superconducting wire from the superconducting state to the normal conductive state. That is, quenching occurs. It is therefore desirable that the shifting of the superconducting wire by the compression force be prevented.